1. Field of the Invention
The present invention relates to a direct fuel injection type spark ignition internal combustion engine in which fuel is directly injected into a cylinder of the engine onto a surface of a recessed dish portion formed in a top portion of the piston. The fuel adhering to the surface of the dish portion is evaporated to generate a combustible gas which is ignited by a spark plug.
2. Description of the Prior Art
Direct fuel injection type spark ignition internal combustion engines are disclosed, for example, in Japanese Utility Model Publication SHO 61-173728. This publication discloses a piston/cylinder arrangement with a deep recessed dish portion and a shallow recessed dish portion formed in a top portion of the piston. In this engine, fuel injected at low engine loads flows into the deep dish portion only, and fuel injected at high engine loads flows into both the deep dish portion and shallow dish portion.
However, there are the following problems to be solved with the prior art engine:
(a) At medium engine loads, a relatively large portion of fuel is injected onto a surface of the shallow dish portion. Since the shallow dish portion has a large area, the fuel injected onto the surface of the shallow dish portion is so widely spread that the fuel-air mixture in the shallow dish portion becomes too lean to burn smoothly As a result, flame propagation is not smooth in a medium engine load operation.
(b) At high engine loads, the fuel injected at an early stage of a fuel injection period flows onto the surface of the shallow dish portion. Since the piston is not yet at its top position, a swirl is still weak. Thus, fuel evaporation at the surface of the shallow dish portion is insufficient, and it is difficult to obtain a perfect combustion in a high engine load operation.
(c) At low engine loads, fuel is injected onto the surface of the deep dish portion where the fuel is evaporated with the help of a swirl. Since the prior art engine has no particular means for guiding the evaporated fuel to a spark plug position, improvement in ignition at low engine loads is desirable.